Natural gas processing can range from simple treating and conditioning for pipeline delivery to complex operations needed to meet specifications to produce pipeline sales gas or liquefied natural gas (LNG). Removal of acid gas compounds such as carbon dioxide and hydrogen sulfide as well as removal of organic sulfur compounds, water and mercury is required to meet end product specifications and to avoid product blockages in downstream process equipment. The level of treatment that is required varies according to the treated gas product specifications as well as local environmental regulations.
In some prior art treatment facilities, mercury is removed by a non-regenerable guard bed and carbon dioxide and hydrogen sulfide are removed by a solvent process employing a solvent such as an amine. Water may be removed by a molecular sieve dehydration unit. The more difficult challenge is to remove COS and organic sulfur compounds such as mercaptans, disulfides, and polysulfides. Historically, there are two types of flow schemes to remove these organic sulfur compounds to meet the product requirements. One is a gas-phase treatment, where the molecular sieve unit is designed to remove the organic sulfur compounds into its regeneration gas stream. The regeneration gas is then treated by a physical solvent unit such as a Selexol™ process to produce a sale or fuel gas and an acid gas stream containing the organic sulfur compounds. This acid gas stream, along with the acid gas generated in the amine unit, is then sent to a sulfur plant such as one operating a Claus process for sulfur recovery. The other flow scheme that has been used is a liquid phase treatment, where all or most of the carbonyl sulfide, organic sulfur compounds or both, are allowed to pass through the dehydration unit. If a natural gas liquid (NGL) unit exists, sulfur compounds are expected to be concentrated in the NGL stream. The NGL liquids or an after fractionation stream are then specialty-amine treated specifically for COS removal. Mercaptans are removed by a regenerable caustic process such as a Merox process in which the mercaptans are converted to liquid hydrocarbon disulfides through use of caustics such as sodium hydroxide or ammonia. Finally, the liquid is sulfur-polished to a low sulfur concentration by a molecular sieve unit to remove the remaining sulfur content.
The gas phase treatment option requires the expense of a sulfur plant while the liquid phase option requires an NGL unit. The liquid phase scheme also has an unattractive element in the caustic-based treatment and its associated spent caustic disposal problem. A sulfur plant is quite costly and it is only justified when the sulfur level is high. The use of an NGL plant is only justified when the gas is rich in C2, C3 and C4 components. If the gas processing operator receives the feed gas from different supply sources, the feed gas sulfur level and/or its hydrocarbon contents may vary from time to time, which would not mean that neither a sulfur plant nor an NGL unit would be justified. Due to the shortcomings of the existing systems, an alternative sulfur capture technology is needed. An ideal solution would be to turn these sulfur compounds into either a solid or a liquid form so that they could be physically transported out of the gas processing facility.